In writing any scientific work it is difficult to decide what background knowledge one ought to assume in potential readers. This difficulty is particularly acute when, as in this case, the book is of an interdisciplinary nature and deals with topics which belong properly to physics, chemistry and, to a certain extent, biology. When in doubt I have decided to assume ignorance rather than knowledge. For example, the text is sprinkled with diagrams illustrating the structures of chemical compounds as it is likely that readers with a physics background will be unable to deduce these structures from the names of the compounds. On the other hand, the derivation of formulae, the origins of which are readily available in common text books, has usually been omitted. When derivations are not so easily come by, they have been given. This is true, for example, in the case of the basic expression for the refractive index of a material as experienced by neutrons. I have been unable to find a derivation of this expression in any of the various books on neutron diffraction which I have examined.

The study of thin organic films has expanded enormously in recent years and it has been necessary to be very selective in order to prevent this work degenerating into a bibliography. I have attempted to discuss material in which structure and order are dealt with and to ignore the many papers, interesting from other points of view, in which these matters have not been mentioned or are given only a minor place.